Liquid membrane processes are known for the removal of soluble materials from solution, for example, see U.S. Pats. Nos. 3,389,078 and 3,410,794, wherein hydrocarbons are separated from solution by means of a liquid membrane technique which comprises contacting said hydrocarbon-containing solution with an oil-in-water emulsion, said oil being a solvent for said hydrocarbon and said hydrocarbon being capable of permeating through the water phase of said emulsion. In these processes permeation of the hydrocarbon from solution into the internal phase of the emulsion continues until the concentration of said hydrocarbon in the solution and in the internal phase is equal. At this point there no longer exists a driving force for the hydrocarbon to permeate through the external phase of the emulsion into the internal phase. Thus processes of this nature are limited by the solubility requirements of the internal phase of the emulsion.
In U.S. Pat. No. 3,779,907 dissolved materials are removed from aqueous solution by the reverse method of contacting a water-in-oil emulsion with the solution and allowing permeation of said dissolved materials through the nonaqueous external phase into the aqueous internal phase of the emulsion. In this patent, however, the dissolved species continue to permeate through the oil phase, i.e. external phase of the emulsion, due to the fact that in the internal phase of the emulsion a reactant is present which converts the dissolved species to nonpermeable materials. In effect, the driving force is maintained by converting the permeable dissolved species into another form. This process, however, has also been found to be limited by the fact that certain conversions in the internal phase are not essentially complete, especially when the means of conversion in the internal phase is precipitation. Certain precipitating agents while effectively forming precipitates with the permeable species, form precipitates which are themselves slightly soluble. Thus, the permeable dissolved species is not completely removed from the aqueous solution but only removed down to a level approximately equal to the concentration of said permeable species soluble in said internal phase of the emulsion, i.e. the concentration of said permeable species due to the slight solubility of the precipitate.
It has now been found and is herein claimed as an invention that this heretofore limit on the liquid membrane processes can be overcome by maintaining the concentration of the precipitating agent at a level whereby due to the common ion effect the solubility of the permeable species in the internal phase is suppressed.